1. Technical Field
The present disclosure relates to a driving circuit. More particularly, the present disclosure relates to a driving circuit for LEDs.
2. Description of Related Art
With the rapid development of photoelectric technology, the industry in recent years has developed many kinds of innovative illumination equipments, among which light-emitting diode (LED) lamps obtain widespread attention. Luminous efficiency and durability of the LED lamps are superior to traditional incandescent tubes, and the LED lamps are eco-friendly in manufacturing. As such, the LED lamps are more welcome in the era of energy saving and environmental protection.
Concerning the lighting system composed of LED lamps known as featuring high efficiency and long durability, conversion efficiency and power factor (PF) are two main factors to achieve the high efficiency on the LED lamps. The conversion efficiency is referred to how much input power is actually transferred to LEDs during the process from the alternating-current (AC) power input to the LED output. The conversion efficiency is higher when a higher proportion of the input power is communicated to the output power.
The power factor is related to the real power and the reactive power of a power signal. The power company usually provides a three-phased AC power signal with a household voltage ranged from 110V-220V and an alternating frequency ranged from 50 Hz-60 Hz. In general, an instantaneous power consumption of a resistive load is the product of voltage and current (i.e., P=VI). However, a pure inductive load or a pure capacitive load may cause a phase difference of 90° between current and voltage, and the phase difference will result in a loss of real power. The instantaneous power consumption can be calculated as the following:
P=VI cos θ, in which I represents the current, V represents the voltage, and θ represents the phase difference between the current and the voltage.
In addition, the power factor can be calculated as follows:
      PF    =                  VI        ⁢                                  ⁢        cos        ⁢                                  ⁢        θ            VI        ,in which PF represents the power factor, I represents the current, V represents the voltage, and θ represents the phase difference between the current and the voltage. As shown in the expression above, when the phase difference between current and voltage is 90° (e.g., when the load is a pure inductive load or a pure capacitive load), the power factor will be substantially decreased to zero.
Because the LED has characteristics of inductance and capacitance, it will result in the phase difference between the input voltage and the input current, and also result in the declination of the power factor. Therefore, the improvement of the power factor is required to achieve energy-saving, and it is necessary for the driving circuit to employ a power factor corrector. However, driving circuits which have the power factor corrector usually suffer from the current ripple problem and the flicker problem, which makes the LED current unstable and declines the quality of the emitting light.